Recoil isolator for machine guns and the like



RECOIL ISOLATOR FOR MACHINE GUNS AND THE LIKE Filed Dec. 14, 1966 Sheet ofL ATTORNEY Feb. 4, 1969 A. s. WHITEILHLL 3,425,318

RECOIL ISOLATOR FOR MACHINE GUNS AND THE LIKE Filed Dec. 14. 1966 Sheet 2 of 5 77/75 g 33 3 g 3/ 36 5 5 f, 3 7 I DEFL EC r/a/v g 34 INVENTOR W 5M 40W ATTORNEY Feb. 4, 1969 A. s. WHITEHILL 3,425,318

RECOIL ISOLATOR FOR MACHINE GUNS AND THE LIKE INVEN'T OR Sheet 2 of 3 ATTORNEY ABSTRACT OF THE DISCLOSURE Recoil is isolated by supporting the gun of mass m and firing frequency f for bodily movement relative to a supporting structure and by connecting spring means of spring constant k in recoil force resisting relation between the gun and the supporting structure, the natural frequency i of the gun on said supporting means as given by the formula being a small fraction of the firing frequency f, whereby the transmissibility of the recoil force to the supporting means is substantially less than the recoil force.

This invention is intended to isolate machine gun recoil from the supporting structure. Isolation is of particular importance in machine guns carried by aircraft.

In the drawing, FIG. 1 is a diagrammatic side elevation of an aircraft machine gun, FIG. 2 is a section on line 22 of FIG. 1, FIG. 3 is a sectional elevation of one of the recoil isolators, FIG. 4 is a bottom view of the FIG. 3 isolator, FIG. 5 is a section through a modification of the recoil isolator, FIG. 6 is a diagram of the recoil forces, FIG. 7 is a force-deflection diagram of the recoil absorbing springs, FIG. 8 is a force-deflection curve for the springs of a modification, FIG. 9 is a side elevation of a recoil isolator which operates upon the principle of vibration isolation at high firing rates and upon the principle of energy absorption at low firing rates, FIG. 10 is a diagram of the recoil forces, FIG. 11 is a vibration transmissibility curve and FIG. 12 is a diagram illustrating the operation at high and low firing rates.

In the drawing, 1 indicates the gun having multiple barrels 2 and 3 indicates the aircraft frame or other supporting means on which the gun is supported. At the front of the gun, the aircraft frame has a depending supporting yoke 4 having arms 5 straddling the gun and each provided with a slot 6 pinned to a rod 7 extending from the body 8 of a recoil isolator fixed to the gun. The rear of the gun is supported by a hanger 9 having a ball 10 at its lower end slidably received in guides 11 on the gun. This structure holds the gun in substantial alignment with the air frame while permitting bodily movement of the gun lengthwise of the gun under recoil forces. The gun and its supporting structure are or may be of common construction. The structure of the gun, the number of barrels, and the supporting structure may Vary widely. In all instances the gun is a self contained unit and is movable relative to the supporting structure.

In the recoil isolating structure shown in FIGS. 3 and 4, the rod 7a which corresponds to the rod 7 in FIG. 1 has an enlarged head 12a pinned to one of the arms 5 of the yoke 4. The rod 7a is slidably received in a boss 13a on the body 811 and has a shoulder 14a within the body 8a engaging a washer 15a at one end of a coil spring 16a. At the other end of the coil spring 16a is a thrust washer 17a transferring the pressure of the spring 16a to an annular washer 18a positioned in a nut 19a by a snap ring 20a. The nut 19a is screwed into the housing 8a to comnited States Patent 0 "ice press or preload the spring 16a and is locked in adjusted position by a pin 21a. Between thrust washers 22a and 23a is a laminated elastomeric sandwich 24a comprising a plurality of layers 25a of elastomer and sandwiched between and bonded to metal washers 26a. A self locking nut 27a takes up any slack between the various washers associated with the rod 7a. At the bottom of the housing 8a is a mounting pad 28a for attachment to the gun. When attached, the housing 8a is effectively, rigidly connected to the gun, while the rod 7a is rigidly connected to the airframe. TWo of the FIG. 3, 4 isolators are used in the FIG. 1 installation.

The operation of the recoil isolator of FIGS. 3 and 4 is illustrated in FIGS. 6 and 7. FIG. 6 shows at 29 the recoil force accompanying each shot. The recoil force is of large magnitude of short duration. Even for machine guns firing at high rates (e.g. 6,000 shots per minute), the duration of the forces 29 is only about 10% of the time interval between shots. This means that the average force exerted by the recoil is represented by line 30. The spring 16a is preloaded so that the initial compression of the spring indicated by numeral 31 in FIG. 7 is only slightly less than the average recoil force indicated by the numeral 32 in FIG. 7. Until the recoil force indicated by the numeral 29 exceeds the average indicated by the numeral 30, the gun remains in fixed relation to the airframe. Upon exceeding the threshold 30, the spring 1611 is thereafter deflected along line 33. The deflection along line 33 is at a relatively soft or gradual rate and provides a substantially linear deflection as given by the formula force equals k (deflection) when k is the spring constant of the spring. The steep line 34 represents the deflection of the rod 7a and the associated attached parts. If these attached parts were perfectly rigid, the line 34 would be vertical. Since the forces 29 occur at a repetitive rate, the isolation of the forces 29 from the airframe structure is comparable with that required for isolating vibrations indicated by dotted line 35. For this purpose, the stiffness of the spring 16a should be related to the mass of the gun 1 in such a manner that the natural frequency of vibration of the gun is substantially less than the firing rate frequency. The transmissibility of the vibrations of force F is given by the formula Transmissibility equals where f is the firing rate frequency and f is the natural frequency of the gun when supported by the springs 16a as given by the formula where m is the mass of the gun. Note, the transmissibility of the recoil force to the airframe is a fraction which depends upon the firing rate frequency f, the spring constant k, and the mass of the gun.

By making the natural frequency of the gun f 10% of the firing rate frequency, the transmissibility of force from the gun to the airframe becomes (in the absence of friction or other damping) A of the recoil force originating in the gun. As a numerical example, in a machine gun firing at the rate of 6,000 rounds per minute, the forces 29 might have a peak magnitude of 32,000 pounds. Since this force would act only about 10% of the time, the average force transmitted to the airplane indicated by 30 in FIG. 6 would be 3200 pounds. With the natural frequency of the gun on the springs 16a being 10 cycles per second, the transmissibility to the airframe would be 3200 poundsi320 pounds During the firing of the gun, the springs 16a operate between points 37 and 38 in FIG. 7.

The effect of the elastomeric sandwich 24a, 25a, 26a is to cushion impacts and to damp vibrations.

FIG. shows a recoil absorber for use in guns having two firing rates. For example, the gun of FIG. 1 might be fired at 6,000 rounds per minute and at 3,000 rounds per minute, Each round would give rise to the, same forces but the firing frequency would be different (100 cycles per second for 6,000 rounds per minute and 50 cycles per second for 3,000 rounds per minute). In order to achieve comparable isolation, when fired at 3,000 rounds per minute, the recoil absorber should have a softer spring. Also, when fired at the lower rate, the average forces such as indicated at line 30 is only half as great. In order to meet these conditions, the recoil absorber of FIG. 5 has two springs in series, a spring 16b of the same stiffness as spring 16a, and a spring 116/) considerably softer than the spring 16b. The spring 16b is telescoped over a sleeve 41 slidable on the rod 7b which corresponds to the rods 7 and 7a. The spring 16b is compressed between a flange 42 on the sleeve and a flange 43 on a bushing 44. The spring 116b is telescoped over a sleeve 45 slidable on the rod 7b, and is compressed between a flange 46 on the sleeve '45 and the flange 42 on the sleeve 41. A laminated elastomeric sandwich washer 47 consisting of layers 48 of elastomer sandwiched between and bonded to washers 49 is arranged between the flange 43 on the bushing 44 and a washer 50 held in place in the body 8b by a snap ring 51. A similar laminated elastomeric washer 52 is arranged between the flange 46 and an inwardly extending flange 53 on the housing 8b. The self locking nut 27b, when tightencd, preloads the springs 16b and 116b to a force slightly less than the average force exerted at the slower firing rate. The preload is indicated by the numeral 54 in FIG. 8. When firing at the slower rate, the springs 16b and 116b act in series and the operation is between points 55 and 56 in FIG, 8. Upon firing at the higher rate, the higher average force causes the end 57 of the sleeve 45 to bottom on the flange 42 of the sleeve 41. Thereafter, the spring 116b is ineffective and the entire cushioning action takes place in the spring 16b with the operation being between points 58 and 59 in FIG. 8.

The isolation of recoil by vibration isolation principles is applicable to high firing rates. When a gun must fire both at a high firing rate and at a very low firing rate, the recoil at the high firing rate may be isolated by vibration isolation principle of energy absorption. FIG. 9 shows a recoil isolator for a gun having a high firing rate of 2,000 rounds per minute and a low firing rate of 500 rounds per minute. At the high firing rate, the recoil is isolated by a linear spring which provides a natural frequency of the gun on its supporting means much lower than the firing frequency and thereby obtains isolation of the recoil forces in the same manner as the previously described constructions. At the low firing rate, although the linear spring is still part of the suspension, the recoil forces are absorbed primarily by highly damped elastomeric springs.

In the FIG. 9 suspension, there are brackets 60, 61 to be fixed to the gun and a rod 62 having an eye 63 to be fixed to the air frame. The bracket 60 is telescoped over a cylinder 64 and is clamped against a shoulder 65 by a jam nut 66. The bracket 61 is fastened to the isolator by a cylinder 67 screwed into the bracket until a shoulder 68 seats against the bracket. After installation, the cylinders 64 and 67 are rigidly fixed to the gun so that when the gun is fired to the right, the recoil is to the left as indicated by the arrow in FIG. 9.

At the high firing rate, for example 2,000 rounds per minute, the recoil forces shown at 70 in FIG. occur at a relatively high frequency (33+cycles per second) and the recoil forces can be resolved into a steady force indicated 'by line 71 and an alternating force" indicated by lines 72. Because of the relatively high frequency of the alternating force 72, it can be isolated from the air frame by vibration isolation principles. However, when the gun is firing at its low firing rate, (e.g. 500 rounds per minute) the recoil forces are so widely separated or occur at such a low rate, as indicated by the interval between the peaks 70 and 73, that isolation of the recoil by vibration isolation techniques is impractical.

In the recoil isolator of FIG. 9, the structure for isolating the high firing rate recoil forces comprises a coil spring 74 seated at opposite ends of flanges 75 and 76 on sleeves 77 and 78 slidable on a reduced section 79 of the rod 62. The coil spring 74 is a linear spring having a spring rate indicated by dotted line 80 in FIG. 12. This spring rate is such that the natural frequency of the gun when mounted on the spring has a frequency much lower than the firing frequency. For example, referring to FIG. 11, where 81 indicates the recoil input force, 82 indicates the natural frequency of the gun when supported on the spring 74 and 83 indicates the firing frequency, the transmission of vibration to the air frame is indicated by curve 84 from which it can be seen that at the firing frequency, the transmission to the frame is much less or of much smaller magnitude than the recoil force. The curve 84 is in accordance with standard vibration isolation techniques Where the vibration input force is magnified until the vibration frequency reaches the resonant or natural frequency of the suspension and then declines as the vibration frequency becomes rogressively greater than the natural frequency, The curve, FIG. 11, also shows the reason that a suspension consisting solely of the spring 74 would be unsatisfactory for the lower firing rate. At a frequency one fourth that indicated by the numeral 83, the firing frequency would be that designated by the numeral 85 and the recoil forces, instead of being isolated from the air frame, would be magnified or relatively slightly attenuated. Accordingly, while a gun suspension consisting solely of the coil spring 74 would be effective to isolate the recoil at the high firing rate, it would be totally unsatisfactory at the low firing rate.

The coil spring 74 is preloaded or precompressed to the stress indicated by numeral 86 in FIG. 12. This preload is equal to or slightly less than the average force 71 exerted by the gun at its high firing rate. The preloading of the coil spring 74 is through a metal sleeve 87 arranged between a nut 88 on rod 62 and the flange 75. Adjacent the bracket 60, an elastomeric sandwich mounting 89 is arranged between flange 76 and a flange 90 on a bushing 91 seated against shoulder 92. A similar elastomeric sandwich mounting 93 is arranged between the flange 75 and a flange 94 on a bushing 95 surrounding the sleeve 87. Each of the elastomeric sandwich mountings 89 and 93 comprises a plurality of discs 96 of elastomer sandwiched between and bonded to metal plates 97. The elastomer is preferably selected or compounded to have high hysteresis or internal damping. The thickness of each of the elastomeric bodies 96 is such that the elastomer is extremely rigid in compression stress. Under no load conditions shown in FIG. 9, the preload force of spring 74 holds the flange 76 against shoulder 101 on the rod 62 and holds the sleeve 87 against the nut 88. The sandwiches 89 and 93 are not preloaded by the spring 74.

Upon firing of the gun at the high firing rate, the average force exerted by the recoil as indicated by the numeral 71 in FIG. 10 causes the gun to move to the left as viewed in FIG. 9 a distance sufficient to overcome the preload of the spring 74 so that the deflection of the spring 74 is in the range between the numerals 99 and 100 on FIG. 12. Under this condition, the brackets 60 and 61 and the associated cylinders 64 and 67 move to the left relative to the rod 62. The flange 76 is seated against shoulder 101 so that this movement causes compression of the spring 74 by a force exerted from the shoulder 69 through the flange 94 and the elastomeric sandwich 93 to the flange 75 at the right hand end of the spring 74. This causes the compression in the spring 74 to vary between points 99 and 100 in FIG. 12 and causes reduced transmission of the recoil forces to the air frame in accordance with the vibration isolation principles. The reduction in transmission of recoil forces is comparable with that obtained in the FIG. 1-4 construction.

When the gun is used at the low firing rate, the elastomer sandwiches enter significantly into the operation and are used to increase the energy absorption so that during the interval between each shot, the gun returns to a stable position. At the slow firing rate, the maximum excursion of the gun is considerably greater than at the high firing rate. For example, at the high firing rate, the excursion of the gun might be inch while at the low firing rate, the excursion might be il /2 inch. At the high firing rate, the recoil impulses are treated as a high frequency vibration. At the low firing rate, each recoil impulse is treated separately and its energy is dissipated or absorbed during the interval between shots.

At each recoil impulse during low rate firing, the gun and its associated brackets 60, 61 and cylinders 64 and 67 move bodily or as a unit to the left as viewed in FIG. 9 but a considerably greater distance than at the high firing rate. The initial recoil impulse causes the elastomeric sandwich 93 to be compressed at a spring rate indicated by line 102. Under static or conditions or a slowly applied load, the elastomeric sandwich would have a load deflection curve indicated by line 103. Under dynamic conditions or rapidly applied loads, the elastomeric sandwich is much stiffer as indicated by the difference between the lines 102 and 103. This initial deflection continues until reaching the preload force indicated at 104, the same force indicated by the numeral 86. Thereafter, further deflection is a combination of deflection of the elastomeric sandwich 93 and of the steel spring 74. This results in a curve 105 somewhat above the load deflection curve 80 of the coil spring. The reason for the difference in behavior of the load deflection curve of the spring 74 and the elastomeric sandwich 93 is that the spring 74 has negligible internal damping while the sandwich 93 has substantial internal damping which has the effect of increasing the stiffness for rapidly applied loads. The recoil deflection continues until reaching point 106, at which time the energy of the initial recoil impulse has been stored in the spring 74 and in the elastomeric sandwich 93. Some part of the initial recoil impulse has been dissipated by the internal damping of the elastomeric sandwich. At point 106, the shoulder 98 is substantially spaced from the flange 90 and the flange 94 is substantially spaced from the nut 88. The coil spring 74 is compressed between flanges 75 and 76 and the elastomeric sandwich 93 is substantially compressed between the flanges 94 and 75. This movement creates a substantial clearance between the sleeve 87 and the flange 75 and the nut 88. The position of the shoulder 98 at the end of the first recoil impulse is indicated by the dotted line 107 and the corresponding position of flange 94 is indicated by dotted line 108. 'During this entire movement, no change has taken place in the position of the flange 76 or the rod 62 or the elastomeric sandwich 89.

During the counter recoil impulse, the energy stored in the spring 74 and elastomeric sandwich 93 is returned to the gun, causing it (and the associated brackets 60, 61 and cylinders 64, 67) to move to the right from the position indicated by dotted lines 107 and 108. The initial portion of this counter recoil movement is effected by expansion of the spring 74 until the clearance is taken up between the flange 75 and the sleeve 87. This corresponds to movement from point 106 to point 86 along the dotted line curve 80 in FIG. 12. The same internal friction or damping in the elastomeric sandwich 93 which offered increased resistance to compression on the recoil stroke, ofiers increased resistance to expansion in the counter recoil stroke. Accordingly, when the spring 74 reaches the point indicated at 86 and is compressed between the sleeve 87 and the shoulder 101, the elastomeric sandwich 93 has not expanded to fill the space between the flanges 75 and 94 so that no force is exerted on the shoulder 98 opposing counter recoil movement of the gun and the counter recoil force drops to zero along line 109 and may even become slightly negative due to the effect of friction. Following this, the elastomeric sandwich 93 expands to fill the space between flanges 75 and 94 and thereafter contributes to the resistance of counter recoil along a curve generally indicated at 110. Upon reaching the zero deflection point, the further counter recoil movement is along curves which are the mirror image of 102, 104, 105, 106, 80, 86, 109, 110 and corresponding parts of the curve are indicated by the same reference numerals primed. It will be noted that because of some energy absorption, the excursion indicated by numeral 106' is less than the excursion indicated by the numeral 106. Each successive excusion is of smaller amplitude due to energy absorption and by the time the next shot is to be fired, the gun has settled down to a normal position.

The energy absorption is indicated by the area between curves 102, 105, 106, 80, 109, 110 and by the area between curves 102, 105', 106', 80', 109', 110.

In all forms of the inventon, for firing rates at which the recoil is to be isloated by vibration isolation techniques, the natural frequency of the gun on its recoil springs is a small fraction of the firing frequency. This results in isolation of all but a small fraction of the recoil forces.

What is claimed as new is:

1. In combination, a machine gun having a mass m and a firing frequency f, supporting means, the gun being bodily movable relative to the supporting means, means for supporting the gun for bodily movement relative to the supporting means and including spring means of spring constant k and means for connecting the spring means in recoil force resisting relation between the gun and the supporting means, the spring means having a yieldability under recoil force such that the natural frequency f,, of the gun on said supporting means as given by the formula 1 k 1 ks 2.11 i; x a

is less than times the firing frequency 1, whereby the transmissibility of the recoil force to the supporting means is substantially less than the recoil force.

2. The combination of claim 1 in which the recoil force has an average value P and a peak value several times larger than F and in which the spring means is initially prestressed to a force equal to or slightly less than F 3. The combination of claim 1 in which the means for connecting the spring means in recoil force resisting relation between the gun and the supporting means comprises first and second members movable relative to each other, one member being attached to' the gun and the other member being attached to the supporting means, stops on one member in force transmitting relation to opposite ends of the spring means, stops on the other member in force transmitting relation to opposite ends of the spring means, the stops on the members being independently movable relative to each other in the direction to compress the spring means.

4. The combination of claim 3 in which one of the members is a rod and the other member surrounds the rod.

5. The combination of claim 1 in which the spring means comprises soft spring means in series with stiff spring means and means for bottoming out the soft spring means at forces above a threshold whereby for low firing frequencies, the spring constant is lower than for high firing frequencies.

6. The combination of claim 3 in which the spring means is a. coil spring.

7. The combination of claim 6 in which an elastomeric washer is interposed in thrust relation between one of the stops and the associated end of the spring.

8. The combination of claim 6 in which a laminated elastomeric sandwich is interposed in compressive force transmitting relation between each of the stops on one of the members and the associated end of the spring, said sandwiches having high internal damping and serving as energy absorbers at firing frequencies close to or lower than said natural frequency.

9. The combination of claim 6 in which damping means is interposed in compressive force transmitting relation between each of the stops on one of the members and the associated end of the spring as for absorbing energy at firing frequencies close to or lower than said natural frequency.

10. In combination, supporting means, a machine gun bodily movable as a unit, means for supporting the gun for bodily movement in the direction of recoil forces relative to the supporting means and including a coil spring and means for connecting the coil spring in recoil force resisting relation between the gun and the supporting means, the means for connecting the coil spring in recoil force resisting relation between the gun and the supporting means comprising first and second members movable relative to each other, One member being attached to the gun and the other member being attached to the supporting means, stops on one member in force transmitting relation to opposite ends of the spring, stops on the other member in force transmitting relation to opposite ends of the spring, the stops on the members being independently movable relative to each other in the direction to compress the spring, an elastomeric sandwich interposed in compressive force transmitting relation between each of the stops on one of the members and the associated end of the spring, said sandwiches having high internal damping and serving as energy absorbers.

References Cited UNITED STATES PATENTS 2,293,069 8/1942 McNeill et a1. 8937 373,640 11/1887 Brill.

575,506 1/1897 Driggs 89-42 X 1,877,839 9/1932 Frommer 89--177 2,010,623 8/ 1935 Bugatti.

2,831,404 4/ 1958 Sampson et al 89-44 X FOREIGN PATENTS 945,904 7/1956 Germany.

OTHER REFERENCES Marks Mechanical Engineers Handbook, Sixth Ed., McGraw-Hill, 1958, pp. 5-100 to 5-101.

BENJAMIN A. BORCHELT, Primary Examiner.

STEPHEN C. BENTLEY, Assistant Examiner.

US. Cl. X.R. 2671, 33 

